Video signals may comprise color, brightness and synchronization information. To display stable video images, a device extracts synchronization information from video signals for proper alignment of the video information, namely, to control image raster scanning. Unfortunately, the synchronization information itself may become corrupt and require correction. For example, video tape recorders (VTR) may generate synchronization errors in video signals that often require TBC.
Traditionally, a digital video system using composite or s-video (separate luma and chroma signals) may implement TBC before or after video signal color demodulation. For TBC after color demodulation, two common methods exist. The first method involves a feedback loop that uses the input and output horizontal synchronization signals (e.g., U.S. Pat. No. 5,600,379 to Wagner). The second method relies only on the horizontal synchronization signal measurements made before decoding (e.g., U.S. Pat. No. 6,300,985 to Lowe). Either method ensures a constant number of samples per line.
With consumer televisions, a phase lock loop (PLL) typically aligns the raster scan signal with the horizontal synchronization associated with the input signal. Additionally the PLL may be a low pass filter (LPF). With appropriate time constants the PLL stabilizes display of images with varying line lengths, often called time base errors (TBE). Most TBE arise from the mechanical playback system of video tape recorders, but can also arise from measurement errors in the display device caused by noise.
VTRs produce three distinct types of TBE. A first type results from uneven friction in ball bearings. The uneven friction stretches the tape and causes small variations in line lengths. A second type of error may generate a much larger variation in line length once per field when the VTR playback/record head goes on or off the tape. Trick modes, such as fast-forward, reverse, and pause, cause a third type of error with an incorrect number of lines per field.
VTRs may use a heterodyning process to produce a stable color signal by demodulating and remodulating the color signal with a more stable clock. The process loses the timing relationship between horizontal synchronization signals and the color subcarrier. Analog televisions overcome this problem by demodulating the color signal with a different frequency than the raster scan. In digital decoders, however, using multiple clocks and analog to digital converters (ADC) is cost prohibitive. One clock frequency, therefore, digitizes the signal. That clock frequency locks to either the horizontal synchronization signal or the color burst signal.
When the clock locks to the horizontal synchronization signal, time base correction is automatic. The clock frequency varies to track the horizontal synchronization signals. This variation, however, is difficult to track using a crystal based oscillator, and therefore requires other oscillators with significantly higher jitter. While the increase in jitter is acceptable for vertically aligning the picture, it causes additional color noise.
A device clock may lock to the color burst reference signal, allowing use of a crystal oscillator. The advantage of locking a clock to the color burst is much lower jitter, which reduces color noise. For heterodyne process signals, however, this results in significant time base errors in the decoder output. Use of either synchronization information before decoding or of a feedback loop (e.g., Wagner) may alleviate TBE.
The feedback loop method uses a circuit after decoding to mimic a typical consumer television PLL. Numerous feedback loops are possible. For example, a device could convert the demodulated signal to an analog signal, then convert it back to a digital signal using a clock locked to the analog horizontal synchronization signal. Another possible feedback loop is to do the same process in the digital domain (e.g., Wagner) by replacing the ADC and digital to analog converter (DAC) with a polyphase filter, or other interpolation algorithms, and also replace the analog phase lock loop with its digital equivalent. Either case finds the desired display rate by resampling the output. This method differs from the decoder method that locks to the horizontal synchronization signal because it first demodulates the color information.
Feedback, however, only corrects errors on subsequent lines, resulting in quality equivalent to that of a typical television. Furthermore, the feedback loop requires equality between the TBC output horizontal scan frequency and the input, thereby eliminating the approach in integrated DTV products that merge the final output scaling and the TBC scaling.
Using the synchronization information before decoding involves a feed-forward approach to resample the data based on the input. This implementation is non-trivial. The prior art using this approach eliminates TBE over single lines. By eliminating the errors over a single line, any noise measurement directly translates into horizontal image jitter. It is an object of the present invention to obviate and mitigate at least some of the above-mentioned disadvantages.